This invention relates to an exhaust gas purifying system including an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine, and more particularly to a technique for judging and diagnosing a deterioration of the exhaust gas purifying catalyst.
It is well known that an adsorption-type NOx trap catalyst is used as an exhaust gas purifying catalyst in an exhaust gas purifying system for an automotive internal combustion engine. A technique for judging a deterioration of the adsorption-type NOx trap catalyst is disclosed in Japanese Patent Provisional Publication Nos. 07-208151 and 11-229849.
In Japanese Patent Provisional Publication No. 07-208151, the technique is proposed as follows: A NOx sensor for detecting a NOx concentration is disposed in an exhaust gas passage downstream of an adsorption-type NOx trap catalyst in order to grasp the deterioration of the NOx trap catalyst which adsorbs NOx in an oxidation atmosphere and releases NOx in a reduction atmosphere. The NOx sensor detects the NOx concentration in the oxidation atmosphere formed after NOx is released in the reduction atmosphere, and then the deterioration of the NOx trap catalyst is judged in accordance with a change of the detected NOx concentration with lapse of time.
In Japanese Patent Provisional Publication No. 11-229849, the technique is proposed as follows: A NOx sensor for detecting a NOx concentration is disposed in an exhaust gas passage downstream of a NOx trap catalyst in order to grasp the deterioration of the NOx trap catalyst which adsorbs NOx in an oxidation atmosphere and releases NOx in a reduction atmosphere. A deterioration of NOx trap catalyst is judged in accordance with an output of the NOx sensor at a time when ambient atmosphere is regulated to a reduction atmosphere under the action of an ambient atmosphere regulating means.